The present invention relates to a method for automatically detecting any longitudinal tear that is generated in a conveyor belt during its operation.
During the operation of a conveyor belt, if an object, for example, a sharp piece of metal, sticks into the belt, the piece could work like a knife to tear the belt in its travelling direction (or longitudinal direction of movement). Longitudinal tear may occur even to steel cord conveyor belts since the cords are embedded in the longitudinal direction of the belts.
To prevent such a longitudinal tear from growing in length, some belts have detectable elements including a linear portion, such as a loop coil or a resonance circuit, embedded in the belt at successive intervals along the travelling direction, and the conditions of such detectable elements are monitored by sensors. A sensor is installed at an external fixed point close to the travel path of the belt to detect a signal generated by each detectable element passing by the sensor point and transmit it to a control unit. When a longitudinal tear occurs in the belt, a detectable element will be broken and the sensor, in turn, will not detect nor transmit a signal. Then the control unit will judge that a longitudinal tear has occurred and will halt the operation of the belt. In this way the longitudinal tear will be controlled and held to a minimum. The belt may be placed in service again after a minor repair. A technique for such detection of longitudinal tear is disclosed, for example, in Provisional Patent Publication No. SHO-62-285813.
In recent years there have been provided some conveyor belts having a pair of the above-mentioned detectable elements embedded at each point. This arrangement is intended to extend the service life of each pair of the detectable elements and, in turn, to enhance their reliability since the detectable elements are usually made of a low strength wire that will reliably break whenever a longitudinal tear occurs, and as such the detectable elements may break due to bending fatigue or the like without occurrence of any longitudinal tear. Thus even when one of the two detectable elements is broken, the remaining element will be sufficient to maintain the same detection performance as that of two normal (unbroken) elements. To assure that proper judgment is made stably regardless of the number of detectable elements, the signal from each pair of elements is amplified to a fixed saturation level; the signal is thus treated as an on-or-off signal.
In the conventional method in which pairs of detectable elements are embedded in a conveyor belt to detect any longitudinal tear therein according to the presence or absence (on or off) of a signal from each pair of detectable elements, any abnormality of a pair of the detectable elements cannot be detected until both the elements are broken, and will be detected only after both the elements are broken. This arrangement has the following disadvantages:
a) As it is not possible to detect a break of one element alone, and hence it is not possible to repair until both elements are broken, the time would eventually come when both detectable elements of a pair are broken unless the detectable elements have an exceptionally long service life. When both of the detectable elements of a pair are broken, in contrast with the case of a break of a single element, the control unit will be forced to judge that a longitudinal tear occurred in the belt and, in turn, to stop the operation of the conveyor regardless of the circumstances. Even if the case is actually a break of a pair of detectable elements due to fatigue or the like rather than a longitudinal tear and it is not desirable to halt the conveyor operation due to nonessential reasons, the operator of the conveyor will be forced to halt the conveyor and inspect it. In the case of a genuine longitudinal tear, it is natural to halt the conveyor and repair the belt without delay. In the case of a break of detectable elements, stoppage due to, say, a malfunction of the detector, and inspection will reduce the efficiency of production significantly.
b) If both detectable elements of a pair are broken without any longitudinal tear, except when they can be repaired immediately, the conveyor must be operated without any information on the conveyor portion in which the affected elements are embedded until the elements are repaired during the next outage. The frequency of periodical outage of a conveyor for maintenance is about once per month in many cases, thus blind operation may continue for nearly thirty days. The conventional method, therefore, may bring about a period of very low reliability of longitudinal tear detection while it is intended to improve the same reliability.